This question usually elicits lots of "pencil and paper is all you need responses", so I take the view that there are two camps on this.
The software I have seen online, like GrowVeg, takes quite a lot of time to set up and focuses on plant spacing and laying out your veg patch like an ornamental border.
People who have been doing it for years know that they need a dozen Sprouts plants and the like ... and for them I doubt that a 3D design plan of plantings is required.
I think the most important thing is to keep records of Planned and Actual dates for sowing, Dates Planted (if relevant), First and Last harvest, and Qty of plants. Plus a Notes column for "Too few" / "Too many" / "Family liked/hated". That will then help with planning in future years, in particular knowing when a crop finishes such that another crop can be scheduled to follow it. If you sow and plant (rather than sow direct) then you can be raising plants in advance of the plot being free - but you need the historical record to tell you when that will be! - sow-and-plant typically gives you a 6 week buffer.
I think for Newbies software that has some knowledge of the times for sowing of crops, and also how long they are likely to be in the ground, is a great boon in planning. It is also likely to alert to crops planted in the wrong place for crop rotation, and what soil treatments to consider, and so on. "hand-holding" mode
All of which the folk who have been doing it for year already know and for them Pencil and Paper is easy because they will have few/no changes of mind and thus few/no rubbings out - which is where software gains hugely allowing multiple versions to be tried / kept / compared, and things to be rearranged, or tried in different ways, much more effortlessly than pencil and rubber.
I am now at the stage, after half a dozen years of growing veg here, of just doing it to the same plan / pattern (pretty much ...) as previous years, so I just have a list, organised by 1st and 15th of the Month for my approx dates, of what to sow and when.
Regardless of planning, weather will mean that things need to be changes, are earlier / later than expected ... so my personal view is that these visual planting planners are more faff than they are worth. Just knowing that you want a dozen Sprouts plants or to sow a row XXX yards long, and that you need to sow them approximately on XXX date is sufficient. What you do need to know, first time around, is how much space all those plant choices, collectively, will take up. Sprouts, in particular, need a lot of space per plant, and it may well be that, for the newbie, a visual design tool solves that question for them. No sense buying seeds, and planning, for a huge harvest if you don't have the space to grow all those plants
What I have not seen on the forums is people saying that they find GrowVeg, or the like, useful year after year. No doubt some will now tell me that they do?!!
I have heard of this alternative planner, which I believe is free. No idea if it good/bad/indifferent, or suitable for UK even
http://www.mountainplain.com/mykitchengarden/My_Kitchen_Garden.html